Some FA's say this...it's not in the stuff the company wants us to say. I have said it once or twice, but it's not in my usual speech. Most don't say it to "prove" anything though - they say it because they want you to pay attention to the demo (or at least not chatter away through it), which is why you probably hear it most often right before the safety stuff gets started.

It gets pretty annoying and frustrating after a while...especially when people are talking and laughing loudly, or when they do stuff like one man on a recent flight of mine: right in the middle of it, he stands up, gets his suitcase out of the overhead bin, opens it up and starts to rearrange everything. Ignoring the fact that he missed it (he was not a FF), and that he was distracting and in the way of others - it's just really rude.

Sentiments like the one you mentioned are usually borne out of that frustration.